Photo-responsive nanoporous liquid crystal polymer films for selective dye adsorption

Photo-responsive nanoporous polymer films (AZOF-R(NC6)) have been developed by a template method based on a hydrogen-bonding supramolecular liquid crystal (LC) and a light-sensitive azobenzene LC crosslinker in this work. Anionic nanopores were obtained after the removal of template NC6 using KOH solution. The AZOF-R(NC6) demonstrates charge-selective dye adsorption and the maximum adsorption capacity for Rh6G is 504.6 mg g−1. The AZOF-R(NC6) film without UV light treatment shows a 32% higher adsorption capacity for Rh6G than the AZOF-R(NC6) film treated with UV light within the initial 10 min. In addition, UV light can trigger the release of the adsorbed dye from the polymer film due to the pore size change arising from the trans–cis isomerization. Besides, the used polymer film can be effectively regenerated using a HCl solution. Such functional polymer films with highly ordered nanopores and photo-responsive properties hold great promise in selective adsorption and mass separations.


Introduction
2][3][4][5][6] For those signicant applications, two key challenges should be addressed in advance.The rst one is to achieve nanoporous materials with tailored pore sizes and programmed surface properties.The host-guest template method based on selfassembly of non-covalent bonds such as hydrogen bonds is an effective strategy to create nanoporous materials with desired pore sizes. 7,8The templates can be easily removed aer solidifying the structures and the obtained pores can reproduce the properties of the templates, thus yielding nanosized pores.Another one is to realize stimuli-responsive capture, transport, or release of cargoes from those nanopores in the materials. 9,10arious external stimuli including temperature, light, magnetic and electric elds, pH and ions have been utilized to realize those functions.Light is an attractive stimulus due to its accurate and remote control. 115][16] Integrating a template method with photosensitive azobenzene to fabricate light-responsive nanoporous materials is a feasible pathway to realize those signicant applications such as selective adsorption and separations.

2−
). Feng et al. 31 developed a nano-ltration lm by a template method based on lyotropic LCs for selective dye adsorption and separation.Introducing light-sensitive azobenzene groups in LCs provides a simple and effective approach to realize the lightcontrolled functions.Kuringen et al. 32 constructed photoresponsive nanoporous polymer lms by using azobenzene crosslinker in the fabrication of smectic LC hydrogen-bonding polymer networks; the number of binding sites and pore size changed under UV irradiation due to the trans-cis isomerization of azobenzene groups.Although some light-responsive nanoporous lms have been developed 33 the photo-controlled capture and delivery behaviors of species from those nanoporous materials are rarely reported.
Recently, we developed a two-dimensional (2D) nanoporous smectic polymer lm based on the hydrogen-bonding supramolecular LC assembly via a template method and the obtained nanoporous functional lm displays anisotropic, size-and charge-selective dye adsorption. 34To further demonstrate the light-controlled dye adsorption capability of the lm, we fabricated a photo-responsive nanoporous LC polymer lm (AZOF-R(NC6)) by using a light-sensitive azobenzene crosslinker in the hydrogen-bonding smectic LC polymer network in this work.The NC6 small molecules (Fig. 1a) serve as hydrogen-bonding acceptors to trigger LC phases as well as templates to create anionic nanopores.The azobenzene crosslinker realizes the light-responsive dye-adsorption function of the lm.Our fabricated AZOF-R(NC6) shows charge-selective dye adsorption thanks to the anionic nanopores with rich negative charges aer KOH treatment.The adsorption of cationic dye Rh6G follows the quasi-second-order kinetic model.Upon UV light irradiation, the smectic layer spacing reduces arising from the pore size change.The absorption rate of Rh6G on the AZOF-R(NC6) is slower than that without UV light irradiation.Besides, the Rh6G-loaded AZOF-R lm can be easily regenerated using a HCl solution.Such functional polymer lms with highly ordered nanopores and light-responsive property hold great promise in selective adsorption and mass separations.

Preparation of the LC polymer lms
The azobenzene LC polymer lm was prepared from a mixture of 6OBA$NC6 (90 wt%) and azobenzene crosslinker A6MA (10 wt%).In preparing the supramolecular complex, 6OBA and NC6 are used in a 1 : 1 molar ratio, which aligns with the stoichiometry necessary for optimal hydrogen bonding.Irgacure 819 (0.5 wt%) was added to initiate the polymerization.p-Methoxyphenol (0.1 wt%) was added to prevent the thermal polymerization.All compounds were dissolved in dichloromethane to form a homogeneous solution and the dichloromethane was then removed by natural evaporation.The monomer mixture was dried in vacuum before use.Azobenzene LC polymer lm was obtained through photopolymerization reaction in a home-made glass cell and the thickness of the lm (5 mm) was determined by the cell gap.The fabrication process of the nanoporous polymer lms was schematically shown in Fig. 1b.The monomer mixture was lled into the cell by capillary suction in the isotropic phase (140 °C), and then cooled down to the liquid crystal phase (90 °C) for the subsequent reaction.To prevent the trans-cis isomerization of azobenzene groups, the reaction was conducted using a UV light equipped with 400 nm cut-off optical lter for 1 h.Then the lm was thermally cured at 120 °C for another 20 min aer UV curing to guarantee the complete reaction.To obtain the functional lm with planar alignments, the glass slides (LC cell) needed to be treated with rubbed polyimide (ZKPI-4220).

Preparation of the NC6-removed lms
The NC6 template was removed by immersing the lm in a KOH solution.The rationale for employing KOH to extract NC6 from the polymer lm is based on the pH sensitivity of the hydrogen bonds present within the NC6 complex.These hydrogen bonds are particularly susceptible to cleavage in basic conditions, effectively breaking at a pH above 8. 35,36 KOH provides a strong alkaline environment which facilitates the disruption of these hydrogen bonds, allowing for the selective removal of NC6.In detail, the pristine lms were immersed in a KOH solution of 0.05 M for 30 min under continuous shaking and then washed with ethanol to remove the residual KOH, and nally dried in vacuum prior to the further use.

Characterization
The hydrogen bonds were characterized using a Fourier transform infrared (FT-IR) spectrometer (IR 200, Thermo Nicolet, USA).Polarized optical microscopy (POM) images were obtained using a Caikon microscope (XPF-500C) equipped with polarization lters.A CK-400 hot stage was used to conduct the thermo-controlled experiments.Differential scanning calorimetry (DSC) was performed using a TA instruments Q2000 calorimeter.The samples were heated or cooled at a rate of 10 °C min −1 with an isothermal equilibration of 2 min aer each heating or cooling ramp.The thermogravimetric analyses (TGA) of samples were performed using a STA-449C (Germany).The samples were heated from 30 to 800 °C at a rate of 10 °C min −1 under a nitrogen atmosphere.The XRD patterns were collected on an Anton Paar Saxsess mc2 apparatus attached to an ID 3003 laboratory X-ray generator (General Electric) equipped with a sealed X-ray tube (PANalytical, l (Cu-K a ) = 0.1542 nm, 40 kV, 50 mA).Adsorption kinetics were studied by UV-vis spectroscopy using a UV-vis spectrometer (Spectrum lab 752s, Shanghai).Other UV-vis spectroscopy experiments were performed using a UV-vis spectrophotometer (UV-1800PC, Mapada Instruments, Shanghai).

Dye adsorption
The dye adsorption performances of the functional LC polymer lms were investigated through batch adsorption experiments.Effects of the UV irradiation and pore charges on the dye adsorption were determined.In a typical run, certain mass of functional polymer lms was added in dye solutions with predetermined concentrations, and then the samples were sealed and placed in a thermostatic shaking bath under 80 rpm and at 25 °C with preset time interval.Absorbance of the solutions before and aer reacting with the lms was measured using a UV-vis spectrophotometer in the wavelength range of 300-800 nm.All the adsorption experiments were carried out at least thrice to ensure the accuracy of data analysis.The concentrations of dyes in water before and aer adsorption was calculated from the UV-vis data.The adsorption capacity (q e , mg g −1 ) was calculated by the following equation: where C 0 and C e (mg L −1 ) are the initial and equilibrium concentrations of Rh6G.V (L) is the volume of dye solutions, and m (g) is the weight of dried lms.The equilibrium concentrations of dyes were measured aer adsorption for 24 h at 25 °C, and the adsorption kinetics experiments were conducted in the time range of 0-250 min.Aqueous samples including the initial solution sample as the zero point were collected and examined at designated time intervals at 25 °C.

Preparation and characterization of the monomer mixture
The hydrogen-bonding supramolecular monomer mixture (6OBA$NC6) and the azobenzene crosslinker A6MA were studied by FT-IR spectroscopy.As observed from Fig. 2a, two typical peaks at 1930 and 2430 cm −1 due to the O-H stretching band and its Fermi resonance reveal the formation of hydrogen bonds between the 6OBA and the NC6 (ref.37) The DSC results show that the monomer mixture exhibits LC transitions during the heating and cooling processes (Fig. 2b).The mesophases are identied by slow cooling from the isotropic phase using POM observation, which forms a smectic C (SmC) phase from 124 to 101 °C.Upon further cooling, a smectic A (SmA) phase is observed as the temperatures range from 101 to 48 °C, and then the crystallization happens at 48 °C.Fig. 2c and d present the POM pictures of the monomer mixture at 115 and 90 °C during the rst cooling process, respectively.The temperaturedependent XRD results of the monomer mixture conrm the presence of the SmC and SmA phases (Fig. 2e).A broad band centered at 14.76 nm −1 can be observed from the XRD pattern at 90 °C probably arising from the intermolecular spacing (0.43 nm) in the SmA phase.A sharp peak centered at 1.56 nm −1 indicates a layer spacing of 4.03 nm, which is almost the same as the molecular length of the 6OBA$NC6 in the stretching conformation (4.01 nm).For the XRD pattern obtained at 115 °C, a sharp peak centered at 1.58 nm −1 shows a lamellar structure with a d-spacing of 3.97 nm, which is a slightly smaller than the molecular length of the 6OBA$NC6.A broad band centered at 14.76 nm −1 results from the intermolecular spacing of 0.45 nm.These results indicate that with increasing the temperature, the molecular arrangement changes from a SmA phase to a SmC phase.The XRD pattern shows that the 6OBA$NC6$A6MA forms a layered structure with almost fully stretched aliphatic chains in the smectic phase.The proposed smectic arrangement of the 6OBA$NC6$A6MA is schematically illustrated in Fig. 2f.2475 cm −1 indicates the cleavage of the hydrogen bonds, while the characteristic peaks of the carboxylate at 1549 and 1395 cm −1 become stronger.This provides a direct evidence that KOH has penetrated the lm network, and broken the hydrogen bonds and deprotonated the acid groups (-COOH) into -COO − groups.The thermal stability of the lms was investigated by TGA technique.The weight loss of the AZO-P lm varies with increasing the temperature and a two-step weight loss is noticeable (Fig. 3d) due to the decomposition of NC6 and the polymer chains, respectively.For the AZO-R(NC6) lm, one-step weight loss representing the decomposition of the polymer chains is observed.This result indicates that the lm has occurred to polymerize and possesses good thermal stability.Fig. 3e and f show the typical SEM images of the surface of the pristine lm and the NC6-removal lm, which depict distinct surface characteristics.The pristine lm exhibits a smooth surface, contrasting with the rough surface observed in the NC6-removal lm, signifying the formation of nanopore structures upon NC6 removal.

Dye adsorption by the functional lms
To ascertain the signicance of the nanoporous structure on the dye adsorption, both the AZO-P and the AZO-R(NC6) lms were used to study the dye adsorption.As observed in Fig. 4a, three cuvettes contain pure Rh6G solution, Rh6G solutions reacted with the AZO-P lm and the AZO-R(NC6) lm, respectively.The Rh6G solution aer adsorption using the AZO-R(NC6) lm becomes nearly colorless, and the dye-loaded lm appears pink, indicating that the AZO-R(NC6) lm can strongly adsorb Rh6G molecules.The maximum adsorption capacity of Rh6G is 504.6 mg g −1 .However, the color of the dye solution aer interacting with the AZO-P lm changes little, showing worse Rh6G adsorption of the lm.Absorbance spectra of the dye solutions aer adsorption with the AZO-R(NC6) lm keeps nearly straight, indicating a large Rh6G uptake of the lm.However, no evident difference in the absorbance spectra is observed for the dye solution adsorbed by the AZO-P lm (Fig. 4a), revealing its inability to adsorb the Rh6G.These results indicate that the nanoporous structure in the AZO-R(NC6) lm plays an important role in the Rh6G adsorption.This is due to that the anionic nanopores is generated aer KOH treatment to remove NC6 template from the lm.The presence of abundant carboxylate (-COO − ) groups endows the lm with sufficient charges and high specic surface area, which greatly benets from the adsorption of cationic Rh6G.Fig. 4c shows the SEM image of the surface of the dye adsorption lm.Aer Rh6G adsorption, surface roughness of the lm increased obviously.Furthermore, the surface exhibits swelling state, indicating the occupation of nanopores by Rh6G molecules.
Competitive dye adsorption experiments were performed to investigate the charge-selective adsorption of the AZO-R(NC6).Solutions consisting of anionic MO and cationic MB dyes, both have a similar molecular size, were dissolved in water to obtain a green solution (the inset in Fig. 4b).The UV-vis spectrum of the solution shows two peaks, one at 465 nm corresponding to MO and another at 664 nm ascribable to MB. Aer interaction for 24 h, a yellow solution and a MB-loaded green AZO-R(NC6) lm are observed, implying selective adsorption of MB onto the AZO-R(NC6).The UV-vis spectra of the solution with AZO-R(NC6) lm show the same absorbance peak of MO as the initial solution, while the MB peak is invisible (Fig. 4b), strongly indicating the charge selective adsorption of MB onto the lm.This is due to that the anionic nanopores are obtained aer the removal of NC6 template.Charge interactions play a decisive role in the dye adsorption.Therefore, the AZO-R(NC6) can selectively adsorb the cationic dye MB instead of the anionic dye MO.
Adsorption kinetics is an important factor that affects the adsorption behavior of an adsorbent for adsorbates.Two classical adsorption kinetics models (the Pseudo-rst-order and Pseudo-second-order models) were utilized to t the obtained adsorption data: Pseudo-rst-order model: Pseudo-second-order model: where q e and q t (mg g −1 ) are the adsorption capacities of Rh6G at equilibrium state and at time t (min), respectively; k 1 (min −1 ) and k 2 (g (mg min) −1 ) refer to the rate constants of the Pseudo-rst-order and Pseudo-second-order models, respectively.The kinetic parameters can be calculated from the slopes and intercepts of the linear plots of ln(q e − q t ) versus t for the Pseudo-rst-order model and t/q t versus t for the Pseudo-secondorder model (Fig. 5).All the calculated results were summarized in Table S1 †.The results show that the higher R 2 value (0.9998) is obtained for the Pseudo-second-order model, and the calculated adsorption capacity (q e,cal = 14.37 mg g −1 ) is closer to the experimental value (q e,exp = 13.74 mg g −1 ), indicating that the kinetics of the Rh6G adsorption onto the AZO-R(NC6) lm follow the Pseudo-second-order model.The strong electrostatic interactions between the cationic Rh6G and anionic nanopores of the AZO-R(NC6) lm contribute to the high adsorption capacity of Rh6G.
To study the effect of isomerization of azobenzene groups on the dye adsorption, two AZOF-R(NC6) lms with the same quality were prepared under dark condition, one was irradiated using 365 nm UV light for 1 h, while the other one was not suffered from UV irradiation.And both two lms were immersed in Rh6G solution (5 mg L −1 ).The Rh6G adsorption at different time was recorded under shaking conditions.As shown in Fig. 6a, the AZOF-R(NC6) lm without UV light treatment shows a 32% higher adsorption capacity for Rh6G than the AZOF-R(NC6) lm treated with UV light within the initial 10 min.This may be ascribable to the isomerization of azobenzene groups which causes a space change of the pores in the AZOF-R(NC6).As shown in Fig. 6b, the d-spacing of the

Photo-responsive dye release
Inspired by the photo-controlled space change of the lm.The photo-responsive release behavior of the AZOF-R(NC6) lm for Rh6G was also investigated.The AZOF-R(NC6) lm was rstly placed in a 100 mg L −1 Rh6G solution under dark conditions to adsorb Rh6G till equilibrium.Then, the lm was irradiated with a UV lamp.The change in absorbance before and aer irradiation was recorded.As shown in Fig. 7, the absorbance of the solution increased aer UV irradiation, indicating that part of Rh6G molecules release into the solution.That is because that the azobenzene groups in AZOF-R(NC6) lm undergo a trans-cis isomerization aer UV irradiation, thus resulting in reduction in pore size and delivering part of the adsorbed Rh6G molecules.It should be noticed that the complete release of Rh6G is not realized owing to that the pore size change triggered by the isomerization is limited, and the space squeezing is insufficient to counterbalance the charge interactions between the dye molecules and carboxylic acid groups.

Desorption and reusability
The regenerability and recyclability of an adsorbent are of importance for its practical applications.Although the photocontrolled dye release is realized, the uptake dyes can not fully release due to a limited structure change of the isomerization.Therefore, a HCl solution was used to regenerate the Rh6G-loaded AZOF-R(NC6) lm (C Rh6G = 100 mg L −1 ).The COO − groups are protonated to produce -COOH groups under acidic conditions, thus giving rise to the release of the Rh6G molecules from the lm.The regenerability is evaluated by the removal efficiency of Rh6G in different cycles.As depicted in Fig. 8a, acid facilitates the release of the dye from the lm.
Regeneration is achieved through treatment with KOH alcoholic solution for obtaining the anionic nanopores.The AZOF-R(NC6) lm exhibits the capacity to adsorb Rh6G from the solution, resulting in a red color of the lm and the solution becoming colorless.Subsequent desorption in HCl allows the release of Rh6G from the lm, causing the solution to turn red while the lm becomes colorless.The FT-IR spectra (Fig. S2 †) validate both the adsorption and desorption processes of Rh6G.Specically, the presence of the -CN stretching vibration peak at 1658 cm −1 in Rh6G signies its adsorption onto the lm. 38pon regeneration, this peak disappears, indicating the successful release of Rh6G from the lm.UV-vis spectra indicate that the previously adsorbed Rh6G is released efficiently.Over 98% removal efficiency is obtained even aer ve cycles of adsorption/regeneration, as presented in Fig. 8b, indicating that our polymer LC lm owns excellent regeneration performances.

Conclusions
Novel photo-responsive nanoporous polymer lms (AZOF-R(NC6)) have been successfully fabricated by a template method based on the self-assembly of the hydrogen-bonding supramolecular smectic liquid crystals and an azobenzene LC crosslinker.The template NC6 could be effectively removed by KOH treatment and anionic nanopores were achieved.The AZOF-R(NC6) lm demonstrates a higher adsorption capacity for cationic dye Rh6G and the maximum adsorption capacity is 504.6 mg g −1 .Adsorption kinetics is well described by the Pseudo-second-order kinetics model.The AZOF-R(NC6) lm without UV light treatment shows a 32% higher adsorption capacity for Rh6G than the AZOF-R(NC6) lm treated with UV light within the initial 10 min due to that the pore size of the lm changes because of the trans-cis isomerization of azobenzene groups.In addition, part of the uptake dyes in the polymer lm can be released thanks to the pore size shrinkage arising from the trans-cis isomerization.Besides, our developed polymer LC lm possesses an excellent regenerability.Such functional nanoporous lms with highly ordered nanopores and light-responsive property as well as excellent regenerability hold great potential in selective adsorption and mass separations.

Fig. 1
Fig. 1 (a) Chemical structures of the monomer mixture containing the hydrogen-bonding 6OBA$NC6 complex and the azobenzene crosslinker A6MA.(b) Schematic illustration of the fabrication process of the nanoporous polymer films.

© 3 . 2 .
2024 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 863-871 | 865 Paper RSC Advances Fabrication and characterization of the pristine and NC6-removed lms The polymer lm with 5 mm thickness was fabricated by UVinitiated photopolymerization of the monomer 6OBA$NC6$A6MA.The rubbed polyimide ZKPI-4220 glass surface was used to produce the uniform planar alignment.Aer UV exposure, polymerization of 6OBA$NC6$A6MA is proved by the disappearance of C]C stretching vibration peak at 1640 cm −1 (Fig. S1 †).The planar alignment of the lm was studied by POM observation.As shown in Fig. 3a and b, obvious bright and dark changes are observable upon rotating 45°, showing a good planar alignment of the lm.The nanoporous lms were obtained through removing the template NC6 using KOH solution since hydrogen bonds could break under an alkaline condition.The FT-IR result veries the removal of NC6, as shown in Fig. 3c, the disappearance of the peaks at 1940 and

Fig. 3
Fig. 3 Characterization of the structure and thermal stability of the polymer films.The POM pictures of the (a) planar aligned pristine film and (b) its 45°rotation view.(c) FT-IR spectra and (d) TGA results of the pristine and NC6-removal films.SEM images of the surface of the pristine (e), the NC6-removal (f) films.The arrow represents the direction of alignment.

Fig. 4
Fig. 4 (a) UV-vis spectra and the corresponding samples (the inset) of Rh6G solutions before and after adsorption by the AZO-P and AZO-R(NC6) films for 24 h.(b) Charge-selective MB adsorption over MO.UV-vis spectra and vials (the inset) of the dye solutions before and after adsorption.(c) SEM image of the surface of the dye adsorption film.

Fig. 6
Fig. 6 (a) Adsorption kinetics of Rh6G onto the AZO-R(NC6) film and the UV-treated AZO-R(NC6) film.(b) Schematic diagram of the isomerization of the azobenzene groups and the corresponding structures of the AZO-R(NC6).

Fig. 8
Fig. 8 (a) UV-vis spectra and the corresponding solution samples (the inset) before and after regeneration.(b) Reusability of the AZOF-R(NC6) film for Rh6G adsorption.